In order to meet the wireless data traffic demand that is on an increasing trend after commercialization of 4G communication system, efforts for developing improved 5G communication system or pre-5G communication system have been made. For this reason, the 5G communication system or pre-5G communication system has been called beyond 4G network communication system or post LTE system. In order to achieve high data rate, implementation of 5G communication system in a millimeter Wave (mmWave) band (e.g., like 60 GHz band) has been considered. In order to mitigate a radio wave path loss and to increase a radio wave transmission distance in the mmWave band, technologies of beam-forming, massive MIMO, Full Dimension MIMO (FD-MIMO), analog beam-forming, and large scale antenna for the 5G communication system have been discussed. Further, for system network improvement in the 5G communication system, technology developments have been made for an evolved small cell, improved small cell, cloud Radio Access Network (cloud RAN), ultra-dense network, Device to Device communication (D2D), wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), and reception interference cancellation. In addition, Hybrid FSK and QAM Modulation (FQAM) and Sliding Window Superposition Coding (SWSC), which correspond to Advanced Coding Modulation (ACM) system, and Filter Bank Multi Carrier (FBMC), Non-Orthogonal Multiple Access (NOMA), and Sparse Code Multiple Access (SCMA), which correspond to advanced connection technology, have been developed in the 5G system.
On the other hand, the Internet, which is a human centered connectivity network where humans generate and consume information, is now evolving to the Internet of Things (IoT) where distributed entities, such as things, exchange and process information. The Internet of Everything (IoE), which is a combination of the IoT technology and big data processing technology through connection with a cloud server, has emerged. As technology elements, such as sensing technology, wired/wireless communication and network infrastructure, service interface technology, and security technology have been demanded for IoT implementation, a sensor network, a Machine-to-Machine (M2M) communication, Machine Type Communication (MTC), and so forth have been recently researched. Such an IoT environment may provide intelligent Internet Technology (IT) services that create new values to human life by collecting and analyzing data generated among connected things. The IoT may be applied to a variety of fields including smart home, smart building, smart city, smart car or connected cars, smart grid, health care, smart appliances and advanced medical services through convergence and combination between existing Information Technology (IT) and various industrial applications.
Accordingly, various attempts to apply the 5G communication system to an IoT network have been made. For example, technologies of sensor network, Machine to Machine (M2M), and Machine Type Communication (MTC) have been implemented by techniques for beam-forming, MIMO, and array antennas, which correspond to the 5G communication technology. Application of the cloud RAN as the big data processing technology as described above could be an example of convergence between the 5G technology and the IoT technology.
In general, a wireless communication system has been developed to provide a voice service as well as securing user's activity. Further, the wireless communication system has gradually extended its service up to a data service in addition to the voice service, and has currently been developed up to the extent that it can provide a high-speed data service.
FIG. 1 is a diagram illustrating the configuration of a wireless communication system. Referring to FIG. 1, a wireless communication system may include a terminal 100, a Radio Access Network (RAN) 130, and a Core Network (CN) 140.
The kind of the RAN 130 is divided depending on what Radio Access Technology (RAT) the RAN 130 uses. Accordingly, the terms “RAN” and “RAT” may be mixedly used. Representative examples of the RAM 130 may be a Global system for mobile communications Enhanced data rates for global system for mobile communications RAN (GERAN), a Universal Terrestrial RAN (UTRAN), and an Evolved-UTRAN (E-UTRAN). In particular, the E-UTRAN is also called a Long-Term Evolution (LTE).
The RAN 130 may include several constituent elements. In FIG. 1, for simplicity, only one constituent element is illustrated in the RAN 130, but several constituent elements may be included in the RAN 130. One constituent element 120 of the RAN 130 that interacts with the terminal 100 may communicate with the terminal 100 through a wireless interface 110. The remaining elements of the wireless communication system may be mainly connected by wire. The constituent element 120 of the RAN 130 that interacts with the terminal 100 through the wireless interface 110 may be called, for example, at least one of an evolved Node B (eNB), a Node B (NB) and/or a Radio Network Subsystem (RNS) including the same, a Base Transceiver Station (BTS) or a Base Station Subsystem (BSS) including the same, a radio access point, a home eNB, a home NB, a home eNB Gateway (GW), and an X2 GW. In the description, for convenience, the term “Radio Access Point (RAP)” may be called at least one of examples of the constituent elements 120 of the RAN 130 enumerated as above or the RAN 130 itself.
The RAP 120 may be composed of one or more cells. The cell manages a specific coverage, and the terminal 100 is served within the coverage of the cell. Here, the cell means a cell of a cellular system, and the RAP 120 means a device that manages and controls the cell. However, in the description, for convenience, the cell and the RAP 120 may have the same meaning. Even in explaining one subject (e.g., embodiment), the cell and the RAP 120 may be mixedly used for convenience.
The CN 140 may include a RAN control element. The RAN control element serves to perform overall control function, such as mobility management, authentication, and security. The RAN control element may be called at least one of a Mobility Management Entity (MME) and Serving General Packet Radio Service (GPRS) Support Node (SGSN), and a Mobile Switching Center (MSC).
If the terminal 100 secedes from the coverage of a serving cell due to movement of the terminal 100 or if it is expected that the terminal 100 secedes from the coverage of the serving cell in the near future, a new cell provides a service to the terminal 100 so that the terminal 100 can seamlessly receive the service. As described above, a process in which the serving cell is changed is called a handover. A cell that provided the service to the terminal 100 before the serving cell is changed is called a source cell, and a cell that provides the service to the terminal 100 after the serving cell is changed is called a target cell.
The terminal 100 measures a signal of a cell and reports the measured signal to the serving RAP 120. The cell from which the signal is measured includes one or more of a serving cell and a neighboring cell. The cell that has received the report may determine a start of a handover on the basis of one or more of reported measurement information and a pre-stored mobility parameter. If the mobility parameter is properly set, the handover can start at a proper time. The mobility parameter is a general term for several parameters. Examples of the several parameters may be a mobility parameter that is used to determine a start of a handover for a cell that uses a specific frequency band as a target, a mobility parameter that is used to determine a start of a handover for a cell of a specific RAN 130 as a target, and a mobility parameter that is used to determine a start of a handover for a specific cell as a target.
FIGS. 2A, 2B, and 2C are diagrams illustrating situations in which a mobility parameter is not properly set to cause a connection failure to occur. A connection failure may occur in the case where a handover does not occur at a time when the handover should be performed (Radio Link Failure (RLF)), or a connection failure may occur during the handover (HandOver Failure (HOF)). In the description, the connection failure may be called an RLF and/or HOF.
FIG. 2A is a diagram illustrating a Too Late Handover (TLH). In FIG. 2A, if a mobility parameter of a RAP 120a is set to have a tendency to start a handover too late, the RAP 120a may cause an RLF 210 through providing a service to the terminal 100 continuously unreasonably even in the case where the serving terminal 100 has already seceded from the coverage of the cell in the RAP 120a. After the connection failure 210, the terminal 100 makes a connection to a cell that is different from the above-described cell. Since the mobility parameter of the RAP 120a is not properly set to cause the too late handover to occur, it is necessary to control the mobility parameter of the RAP 120a. 
In FIG. 2A, the cell that is connected to the terminal 100 after the connection failure 210 is depicted as a cell within a RAP 120b that is different from the RAP 120a, but it is not necessary that the cell becomes the cell within the other RAP 120b. However, if the cell connected after the connection failure 210 and the cell connected before the connection failure 210 are cells in the same RAP 120a, follow-up measures between the RAPs 120a and 120b may not be separately necessary. In the description, a case where the cells that are connected before and after the connection failure are cells in different RAPs 120 is mainly considered as a more general situation, but even a case where the connected cells are cells in the same RAP 120 would not be excluded.
FIG. 2B is a diagram illustrating a Too Early Handover (TEH). In FIG. 2B, a mobility parameter of a RAP 120a is set to have a tendency to start a handover too early. In this case, the RAP 120a unreasonably starts a handover to another cell even in the case where the serving terminal 100 is still within the coverage of the cell in the RAP 120a. Accordingly, an RLF 230 may occur even in the case where it is not long since the handover occurred, or a HOF 235 may occur during the handover. After experiencing the connection failure, the terminal 100 makes a connection to a cell in the RAP 120a again. Since the mobility parameter of the RAP 120a is not properly set to cause the too early handover to occur, it is necessary to control the mobility parameter of the RAP 120a. 
FIG. 2C is a diagram illustrating a Handover to Wrong Cell (HWC). Referring to FIG. 2C, if a mobility parameter of a RAP 120a is set to have a tendency to start a handover to a wrong cell (cell in a RAP 120b), the RAP 120a does not start a handover to a cell (cell in a RAP 120c) that is suitable to actually provide a service to the serving terminal 100, but starts the handover to a preposterous cell. Accordingly, an RLF 260 may occur even in the case where it is not long since the handover occurred, or a HOF 265 may occur during the handover. After experiencing the connection failure, the terminal 100 makes a connection to a cell in the RAP 120c. Since the mobility parameter of the RAP 120a is not properly set to cause the handover to a wrong cell (cell in the RAP 120b) to occur, it is necessary to control the mobility parameter of the RAP 120a. 
After the connection failure 210, 230, 235, 260, and/or 265, the terminal 100 may transmit a reestablishment request to the RAP 120 that includes a suitable cell (no mode change). If a suitable cell is not found for a predetermined time, the terminal 100 may be shifted to an idle mode, and then if the suitable cell is found, the terminal 100 may be shifted to a connected mode.
In the case where the suitable cell is not found for the predetermined time, the connection failure reason may be that the terminal 100 has moved to a shaded area or an internal problem (e.g., security related problem) of the terminal 100, rather than that the handover starts too late, too early, or to a wrong cell. That is, the connection failure may not be caused by problems in setting the mobility parameter of the RAP 120. Accordingly, in this case, it may not be necessary to separately take Mobility Robustness Optimization (MRO) measures.
In contrast, if the terminal 100 transmits the reestablishment request after the connection failure 210, 230, 235, 260, and/or 265, the terminal 100 may take MRO measures under the assumption that the reestablishment request target cell (cell in the RAP 120b in FIG. 2A, cell in the RAP 120a in FIG. 2B, or cell in the RAP 120c in FIG. 2C) is a cell that is suitable to serve the terminal 100 at the time of the connection failure 210, 230, 235, 260, and/or 265. The MRO measurements will be described in detail later.
The assumption that the reestablishment request target cell is a cell that is suitable to serve the terminal 100 at the time of the connection failure may be effective, for example, when the reestablishment is successfully performed. For example, if the reestablishment request is not suitably transferred to the RAP 120 and thus the reestablishment is not successfully performed, the assumption that the reestablishment request target cell is a cell that is suitable to serve the terminal 100 at the time of the connection failure may not be effective.
However, the current MRO measurements are being performed in consideration of the reestablishment attempt itself, without considering the reestablishment result. Accordingly, there has been a need for the MRO measurements in consideration of the reestablishment result.